Inkjet printers are well known. One common type of inkjet printer uses a replaceable print cartridge which contains a printhead and a supply of ink contained within the print cartridge. The print cartridge is not intended to be refillable and, when the initial supply of ink is depleted, the print cartridge is disposed of and a new print cartridge is installed within the scanning carriage. Frequent replacement of the print cartridge results in a relatively high operating cost.
The printhead has a useable life which is significantly longer than the time it takes to deplete the ink within the print cartridge. It is known to refill print cartridges intermittently by creating an opening through the print cartridge and manually refilling the print cartridge with ink.
It is also known to provide an external, stationary ink reservoir, such as a flaccid bag containing ink, connected to the scanning print cartridge via a tube. Such external ink supplies are referred to as off-axis ink supplies.
Extended use of a print cartridge creates certain problems. Applicants have discovered that there is a diffusion mechanism that has the effect of growing air bubbles in the manifold of the print cartridge and even pressurizing the print cartridge. In particular, during operation, cool ink flows into the manifold and is warmed as it flows toward the printhead. The printhead generates heat as its heater resistors are fired to eject droplets of ink from nozzles. For primarily water-based inks, the solubility of air decreases as it is heated. As a result, air is driven out of the solution, and diffuses into any preexisting bubbles in the manifold. Moreover, because the warmed ink is expelled from the nozzles and replaced with cool ink, there is a steady supply of air from the warming of the ink that diffuses into the preexisting bubbles in the manifold. Eventually, the entire manifold will fill with air.
Excessive air in the manifold of the print cartridge acts as a flow restrictor and will eventually cause printhead starvation. More specifically, accumulation of about 1 cc of air in the manifold will cause print quality defects for high density graphics.
Additionally, air also accumulates in the tubes leading to the print cartridge. Excessive air in the tubes will eventually be drawn into the body of the print cartridge, and thermal excursions will cause the air in the body of the print cartridge to expand. This will eventually lead to non-optimal operation of the print cartridge.
Print cartridges having an internal pressure regulator for regulating the flow of ink from an external source into an ink chamber within the print cartridge have been developed at Hewlett-Packard. Print cartridges including such an internal pressure regulator incorporate a diaphragm in the form of a bag. The inside of the bag is open to the atmosphere. The expansion and contraction of the bag controls the flow of ink into the print cartridge to maintain a relatively constant back pressure at the printhead. However, when roughly 5 cc's of air have accumulated in the body and manifold of the print cartridge, the regulator no longer has the capacity to maintain negative pressure. At that point, air in the printhead will render non-functional any pressure regulator internal to, or leading to, the print cartridge. As a result, the back pressure is lost, or the print cartridge is even pressurized, and ink will drool out of the printhead. A drooling printhead can cause permanent damage to the printer. Moreover, a drooling printhead provides unacceptable print quality. Therefore, the accumulation of excessive air in the body and manifold of print cartridges shortens the useful life of permanent and semi-permanent printheads.
Problems with air accumulation may also occur with print cartridges not having an internal regulator. It is not economical to throw away a print cartridge due to the above described air accumulation.
What is needed is an apparatus for inkjet print cartridges which avoids the air accumulation problems described above.